feat(commands): unify into :MdRender <subcommand> dispatcher

[delphinus]

Closes #11.

Summary

• Consolidate the 7 top-level commands into a single :MdRender <subcommand> dispatcher (float, tab, pager, toggle, split, auto on|off|toggle, demo), following the convention used by Trouble v3 / Telescope / Lazy
• Two-level tab completion: subcommands on the first arg, on / off / toggle after auto. :vert MdRender split and other Vim modifiers compose naturally
• Old commands (:MdRenderTab, :MdRenderToggle, :MdRenderSplit, :MdRenderAuto, :MdRenderPager, :MdRenderDemo) stay registered as deprecated aliases that forward to the dispatcher and emit a one-shot `vim.notify_once` warning per session. Slated for removal in a future major version
• <Plug> mappings unchanged — they bind to functions, not commands, so user keymaps don't break
• Vimdoc restructured: single *:MdRender* block with new *:MdRender-{float,tab,toggle,split,auto,pager,demo}* sub-tags, plus deprecated stub blocks so :help :MdRenderToggle still resolves
• README (English + Japanese) command tables and narrative updated, with a backwards-compatibility note

Design notes

Per the issue's open questions (confirmed before implementation):

• Two-level completion implemented, not just first-arg
• :MdRender demo kept as a public subcommand (not hidden)
• No named args (width=120 etc.) this round — open question deferred to a follow-up
• Deprecation warning lands now (this release); old commands will be removed in v4.0.0

The dispatcher lives in a new `lua/md-render/command.lua` module rather than inline in `plugin/md-render.lua` so it's unit-testable (the Makefile runs tests with `--noplugin`).

The repo has no `CHANGELOG.md` (release notes live in git commits and GitHub releases), so the issue's CHANGELOG line is dropped.

Test plan

• `make test` — 500 passed / 0 failed (35 new in `tests/command_test.lua` + 465 existing)
• Headless smoke test: 7 commands register, completion returns expected lists for empty / `auto` / `:vert MdRender` cases, deprecation warning fires exactly once per session via real `vim.notify_once`
• Manual smoke in interactive Neovim against `assets/showcase.md`: `:MdRender`, `:MdRender float`, `:MdRender tab`, `:MdRender pager`, `:MdRender toggle`, `:MdRender split`, `:vert MdRender split`, `:tab MdRender split`, `:MdRender auto on/off/toggle`, `:MdRender demo`
• Tab completion in interactive Neovim: `:MdRender `, `:MdRender auto `, `:vert MdRender `
• Each old command (`:MdRenderToggle`, `:MdRenderSplit`, `:MdRenderAuto on`, …) still works and prints the one-shot deprecation warning the first time it's used
• `:help :MdRender`, `:help :MdRenderToggle`, `:help :MdRender-split` resolve correctly

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